FBI agents are reportedly scrutinising images of a man running from the scene of the Boston Marathon bombings.
The shots, taken by amateur photographer Benjamin Thorndike from his nearby office window, show the young man behaving in stark contrast to those surrounding him.
Thorndike stresses he is not labelling the man a suspect, telling The Boston Globe: “For all I know, this is a guy who was watching the race who got burned.”
However he points out to ABC News: “I was struck by this individual.
“Everyone else in the photo is on the ground, bent over, holding their ears, appearing to be in shock, immobile, but this one individual is exactly the opposite.
“He’s sprinting. His clothes are in tatters. They were melted off, burning off his body, so for all I know he was panicked and fleeing.”
The pictures have been handed over to the FBI, who have spoken to Thorndike several times, Boston CBS reports.
It adds: “The FBI did not reveal anything to Thorndike about the investigation or the running man.”
The FBI are scouring thousands of photographs and video of the scene, where two explosions killed three and injured more than 180.
The before shot shows the bag next to a mailbox along a barricade, while the after shot, taken after the explosion, shows no sign of the bag.
Images of one of the bombs have been released showing the remains of a black backpack which contained the device.
The FBI says it has evidence it was contained in a pressure cooker filled with nails and ball bearings.
Theories also abound about an image of a man standing on the roof of a nearby building, which was posted on Twitter.
Three people, including eight-year-old Martin Richard who was hoping to see his runner father cross the finishing line, lost their lives in the blasts. Krystal Campbell, 29, and Lu Lingzi, 23 were also killed.
While pressure cooker bombs are more commonly used in Afghanistan, Pakistan, India and Nepal; they have been seen before in domestic terror attacks, including the 2010 attempted bombing in Times Square.
No one has claimed responsibility for the bombings and no arrests have been made.